Shielding stepper motor wires

Discussion in 'The Projects Forum' started by leonhart88, Dec 9, 2010.

  1. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Dear All,

    I did do a search on this and found one thread on a similar topic (http://forum.allaboutcircuits.com/showthread.php?t=46447). I believe one of the members in that thread gave me the answer, but I want to double check.

    I have a stepper motor for a peristaltic pump I'm using. The wires it came with were just 22AWG wires (connected to a PCB). This pump was purchased, so I don't know anything about the electronics/PCB. I cut the wires and soldered new wire to extend the distance the motor can be from the PCB. I asked the company before doing this and they said it should be OK (it's an OEM pump).

    However, I'm a bit concerned that the wires may be picking up noise that can affect my results. I would like to put some shielding on the wires, or purchase a shielded cable with 4 conductors in it and use that instead. However, I'm not sure how to connect/solder shielded cable, as I have never done that before.

    From doing some research, I can just purchase a 4 conductor (the motor has 4 wires) shielded cable and solder the cables together, with ONE end of the shielding connected to the PCB ground. Is this correct?

    Thanks,
    Philip
     
  2. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    On a side note....how would I even tell if the un-shielded wires are causing problems?

    I can't really tell if there are problems with the motor's steps, since it seems to be running OK. I do get some variations in my pump results sometimes though...but I have a pretty large setup with many variables that may be the reason for that. I do worry a little that the noise and un-shielded wires may cause the motor to skip steps once in a while. I'll probably remove everything and just test the pump by itself to see if it pumps a consistent amount of water every time. If shielding is a problem with motor control...i should see problems.

    One other oddity I noticed is that once in a while the electronics will not detect a "completion" command from the pump/motor. Every time the motor/pump finishes a command, it's supposed to return a completion message to the electronics, which will then show up on the serial port. Every once in a while this message goes missing though...and I am not sure if this is because of the wires as well.

    I don't know the exact length of the wire...but I made it approximately 3-5 times longer than it was originally. It might be approximately...60-80 cm long?

    Thanks,
    Philip
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Make sure you have decoupling caps on the controller supply.

    For long connections, use twisted pair shielded, use differential signals on each pair (one high, one low). Shielded twisted pair has foil around all conductors which should be grounded on both ends.

    Grounding straps should be flat woven braid cable, typically bare copper, this gives ample current carrying ability, and reduces any skin effect of EMI. Think of desoldering wick, but about 5 or 10 times the size.
     
  4. mtripoli

    New Member

    Feb 9, 2010
    44
    8
    "Sheilding" is often misunderstood. When talking about a cable the rule of thumb is to "ground" at the ORIGINATION end and NO CONNECT at the receiving end. This throws many people, so don't worry if it doesn't make much sense.

    The sheild at the origination end should be grounded; the other end dieconnected. If you ground both ends you create a ground loop. So, connect all SIGNAL GROUNDS, but only connect the SHEILD at one end.

    I can give you a schematic if you need...

    Mike T.
     
  5. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    If you do not have a ground strap, then only ground the shielding at the source end.

    With a good ground strap, grounding both ends is indicated, as the ground strap will show a lower impedance than the shield.
     
  6. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Thanks guys. As I mentioned before...I did not design the PCB. I can only see one ground input (for the power supply ground) connected to the board. Is this the ground I should connect the shielding to? I'll have to splice the wires so that I can solder the shielding onto the ground, as well as have the PCB attached to ground.

    I was actually looking at a cable such as this one:

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=A3223-100-ND

    It has 4 conductors in it, with a braided shield. I'll solder the 4 conductors to the correct stepper motor wires and then solder one end of the shielding to the PCB ground. The other end of the shielding will be left in the cable...connected to nothing.

    Does this sound right?

    Thanks,
    Philip
     
  7. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Also...do you guys happen to know where I can buy smaller quantities of the cables (in Canada or the US)? I don't really need a whole spool...
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    You can do a search for "Shielded Cat5", and probably find lower quantities to fit your need. Cat5 has 4 twisted pairs, total of 8 conductors.
     
  9. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    OK, i'll do that. Thanks a lot for the help, I really appreciate it.
     
  10. shortbus

    AAC Fanatic!

    Sep 30, 2009
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  11. leonhart88

    Thread Starter Senior Member

    Feb 23, 2007
    118
    1
    Thanks.

    So if I purchase a 4 pair cable, do I just leave the other 4 wires I don't need unconnected (I only need 4 conductors)? Should I put some electrician tape or something over the ends?

    Thanks,
    Philip
     
  12. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    You can leave unused pairs disconnected in this application. If the motors draw a lot of current, you could double up wires (between pairs, not one pair each) for better current handling ability.

    By "between pairs", something like Motor+ would be, say green and brown, and Motor - would be white/green and white/brown, for example. That keeps the signals from radiating due to the twists, and reduces the ability for the cable to act like an antenna.
     
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